(1) Field of the Invention
This invention relates to a plastic lens resin having a high refractive index (hereinafter may also be called "refractivity"), low dispersion, and either no or extremely little optical strain.
(2) Description of the Prior Art
Plastic lenses have been finding utility as eye lenses and camera lenses and in optical devices in recent years since they are lighter, less fragile and dyeable compared with inorganic lenses. As a resin employed widely for these applications these days, there is a resin obtained by subjecting diethylene glycol bisallylcarbonate (hereinafter called "CR-39") to radical polymerization. This resin has various merits such as excellent impact resistance, light weight, superb dyeability, and good machinability such as cutting, grinding and polishing machinability. Its refractive index is however small (n.sub.D =1.50) compared with inorganic lenses (n.sub.D =1.52). In order to achieve optical characteristics equivalent to glass lenses, it is essential to increase the central thickness, peripheral thickness and curvature of a lens, thereby unavoidably increasing the overall thickness of the lens. There is hence an outstanding demand for the development of a lens resin having a higher refractive index in order to overcome this inconvenience.
As lens resins which can purportedly materialize higher refractive indexes, there have been proposed urethane resins obtained respectively by a reaction between an isocyanate compound and a hydroxy compound such as diethylene glycol (Japanese Patent Laid-Open Nos. 136601/1982 and 136602/1982), a reaction between an isocyanate compound and a halogen-containing hydroxy compound such as tetrabromobisphenol A (Japanese Patent Laid-Open No. 164615/1983) and a reaction between an isocyanate compound and a sulfur-containing hydroxy compound (Japanese Patent Laid-Open Nos. 194401/1985 and 217229/1985); and S-alkyl thiocarbamate lens resins obtained individually by a reaction between an isocyanate compound and an aliphatic polythiol (Japanese Patent Laid-Open No. 199016/1985).
The S-alkyl thiocarbamate lens resins involve a few problems. They have large polymerization velocities even at relatively low temperatures and substantial difficulties are encountered in controlling the heat during their polymerization. Further, the resulting lenses have great optical strain. It is hence difficult to provide useful lenses.